Skip to main content

Exercise #4 A typical night at Yankee Tank Creek Observatory

4 posts / 0 new
Last post
Ed Wiley_WEY
Ed Wiley_WEY's picture
Exercise #4 A typical night at Yankee Tank Creek Observatory

The observatory is located in my backyard under 4-4.5 magnitude skies in the southern fringe of Lawrence, KS.

Hardware: Shyshed Pod dome, 8" Schmidt-Newtonian with Moonlite focuser, tube is flocked; ST402ME NABG camera with BVI internal filter wheel; Losmandy G-11 (smooth PE, no "76second" non-harmonic error), Sky Commander with DSC "Push-To:" Guiding via a F6 66mm refractor, DSI Pro camera and PhD

Software: CCD Soft V5, SkyX Pro, PhD, AIP4WIN, VPHOT: CCDSoft and SkyX integrate with the Sky Commander.

Routine

1. Open the dome in the afternoon for temperature equilibration

2. Before sundown I fire up the camera and set the temperature

3. At about sundown I take flats

Flat protocols:

     a. Close the dome. It is translucent so there is plenty of ambient light coming through the grey dome.

     b. Point OTA to dome (mostly vertical) and place two 1/8" "white" acrylic plates at the end of the OTA to produce a flat field.

     c. Take 15 each B, V, I integrations, minimum exposure of 2 seconds (usually 15-20 seconds B, 10 seconds V and 2-5 seconds I; depending on light conditions).

     d. follow with flat darks of same integration times of each flat.

4. When fully dark I take bias and darks. I standardize my B and V at 120 seconds, but the "I" integrations vary, so I take darks at 120s.

5. Take the images

6. Calibration protocols:

     a. Images are segregated into folders for each filter.

     b. For images of each filter I do the following using the "Advanced" routines in AIP4WIN.

     c. Flats are calibrated in using the normalize median routine that is supposed to handle slightly different ADU values of

twilight flats.

     d. Darks and bias are used to provide "automatic dark matching."

     e. Each folder is then calibrated using the appropriate flats along with the darks and bias.

7. Post-processing

     a. sort images into separate folders by variable

     b. Using CCDSoft and SkyX: plate solve and stick in OBJECT and CALSTAT Fits keywords and values

8. Analysis: VPHOT

About the only thing out of the ordinary is my peculiar way of taking flats. It is a kind of twilight/dome flat hybrid where I take advantage of the diffuse light coming though the dome coupled with the white acrylic plates as a secondary diffuser. I would be interested in what a real pro thinks of these flats and would be glad to share.

There is also the question of how frequently flats should be taken given the OTA/Camera has not been touched and in an observatory. I would be interested in hearing from Aaron and other on this topic. Probably like everyone else, I have missed taking flats on occasion and ended up either using two-day-old ones or taking them the next evening (cloudy or not).

No radical change in habits, but I am more aware of the need for good flats and darks and will take "covered" darks from now on.

HQA
HQA's picture
twilight flats

[quote=Ed Wiley_WEY]

About the only thing out of the ordinary is my peculiar way of taking flats. It is a kind of twilight/dome flat hybrid where I take advantage of the diffuse light coming though the dome coupled with the white acrylic plates as a secondary diffuser. I would be interested in what a real pro thinks of these flats and would be glad to share.

There is also the question of how frequently flats should be taken given the OTA/Camera has not been touched and in an observatory. I would be interested in hearing from Aaron and other on this topic. Probably like everyone else, I have missed taking flats on occasion and ended up either using two-day-old ones or taking them the next evening (cloudy or not).

No radical change in habits, but I am more aware of the need for good flats and darks and will take "covered" darks from now on.

[/quote]

Your hybrid flats should be fine.  The acrylic plates should be rough and not shiny, as you want to avoid any kind of reflection.  How often you take flats depends on how often things change in your system.  I rarely have a longer interval than about a week, even with a closed system like an SCT, filterwheel, camera.  When imaging at USNO, I took flats every nights because the telescope was an open-tube and we accumulated dust on the filters.  The first time you get burned with a big dust donut that wasn't there a few days ago when you took flats will teach you the proper interval with your system.  I always ratio tonight's flats with the last set that I took to look for changes.

Arne

Ed Wiley_WEY
Ed Wiley_WEY's picture
Flat frequency

Thanks for the reply, Arne:  my optical train is stable over weeks, the only times I break it down is when I switch scopes. I try to take flats for each night of observing. However, I have been known to go up to three nights on a set of flats. Three nights in a row is about the limit here in eastern Kansas, we rarely get more than three in a row. If its has been cloudy for a couple of days I take new flats. I will start comparing my flats to see if I can pick up changes.

Good point about the acrylic plates, mine are smooth-milky. I am off the the store tomorrow to get rough ones, frosty I am guessing but I will see what they have.

Ed

CROA
CROA's picture
Hi, I am following this

Hi,

I am following this conversation with interest,

and I take my Flats a similar way, I keep them

for ca. a week and replace them with new ones,

so far I have no problems with it.

 

  Cheers, Rolf

Log in to post comments
AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484